1. Field of the Invention
The present invention relates to a post-processing apparatus and a method of controlling the same, and more particularly to a technique of controlling a post-processing apparatus that punches a hole at a predetermined position of a conveyed sheet.
2. Description of the Related Art
Conventionally, there has been widely known a post-processing apparatus that punches sheets discharged from an image forming apparatus, such as a printer, a copying machine, and a facsimile machine.
A post-processing apparatus conveys a sheet received from an image forming apparatus along a conveyance passage to a punching unit. In doing this, in a state in which an abutment member is retracted from the conveyance passage of the sheet, the sheet is conveyed by being nipped between conveyance rollers. The abutment member is a member for having a trailing end of the sheet abutted thereon by switching the sheet back thereto, to thereby position the trailing end of the sheet and eliminate skewing of the trailing end side of the sheet. After a leading end of the sheet passes a punching position, the sheet is moved to a center position in a lateral direction (direction orthogonal to a sheet conveying direction), by a shifting operation for shifting the conveyance rollers nipping the sheet in the lateral direction. Thereafter, when the trailing end of the sheet passes the punching position, the abutment member is returned to the conveyance passage. Then, nipping of the sheet by the conveyance rollers is cancelled, and a sheet alignment in the lateral direction is performed using a pair of alignment members. Subsequently, the sheet is nipped by conveyance rollers, and further, the trailing end of the sheet is caused to abut against the abutment member, whereafter punching is performed on the sheet by a punching unit (see U.S. Pat. No. 7,333,767).
According to U.S. Pat. No. 7,333,767, the alignment members are kept on standby at a position distant from the conveying center position in the lateral direction (hereinafter referred to as a “defined center position”) so as to prevent the conveyed sheet from being brought into contact with either of the alignment members. The retracted position of each alignment member is set by taking into account a case where the conveyed sheet is most greatly displaced from the defined center position. In other words, the amount of shift of each alignment member is fixed regardless of the amount of displacement of the sheet, and hence there is a problem that it requires the same shift alignment time irrespective of the magnitude of the amount of displacement of the sheet, which makes it impossible to improve productivity. The shift alignment time represents a time period from a time when a sheet shifting operation is started to a time when skewing is corrected by the alignment members.
As a solution to the abovementioned problem, there is provided a method of setting the standby position of each alignment member closer to the defined center position. In this case, however, when the amount of displacement is large, there is a possibility that the sheet is damaged by being brought into contact with either of the alignment members.